Methods and structure for improving wafer bow control

ABSTRACT

A method for controlling bow in wafers which utilize doped layers is described. The method includes depositing a silicon-germanium layer onto a substrate, depositing an undoped buffer layer onto the silicon-germanium layer, and depositing a silicon-boron layer onto the undoped layer.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

The United States Government has acquired certain rights in thisinvention pursuant to Contract No. F33615-01-02-5705 issued by theDepartment of the Air Force.

BACKGROUND OF THE INVENTION

This invention relates generally to manufacturing of MicroElectromechanical System (MEMS) devices, and more specifically to,manufacturing of a substrate layer for MEMS devices utilizing heavilydoped silicon as an etch stop.

One method for making MEMS devices involves depositing a very heavilyboron-doped silicon layer on a lightly doped silicon substrate wafer.After various patterning steps, part of the substrate is etched away inalkaline etchants such as potassium hydroxide orEthylene-Diamine-Pyrocatechol (EDP), and water, plus a trace amount ofPyrazine. The heavily doped silicon layer is not affected by theseetchants, creating a natural etch stop. In another method, the siliconwafer is bonded to a glass wafer. Prior to bonding, the silicon wafercan be patterned. Additional patterns can be made on the glass wafer.The entire lightly doped substrate is then etched away, leaving only thepatterned, heavily doped layer attached to the glass. The boron dopantconcentration in the doped layer is >1×10²⁰ cm⁻³. At this concentrationthe boron atoms, which are smaller than silicon atoms, cause a shrinkageof the silicon lattice. Thus the doped layer has a high tensile straincompared to the substrate, causing the wafer to bow. The bow is severeenough that many pieces of fabrication equipment cannot handle thewafers. Therefore, additional layers or processes are required tocontrol the wafer bow and create a relatively flat wafer. Two methods,boron-germanium co-doping and a backside tensile layer, have been widelyused for controlling wafer bow. Germanium co-doping and backside tensilelayering are described below in detail with respect to FIGS. 2 and 3respectively.

There are two negative consequences of boron-germanium co-doping thatmake this approach unusable for some devices. One negative consequenceis that the high germanium concentration (>1×10²¹ cm⁻³) degrades themechanical properties of the silicon. An example of this is a high levelof internal damping in a MEMS resonator. Another negative consequence isthat the differing diffusion coefficients of boron and germanium insilicon result in some segregation at the interface between thesubstrate and the doped layer. This segregation creates undesirablestress gradients at the edge of the doped layer.

With regard to the backside tensile stress layer, when a heavily borondoped layer is deposited on a lightly doped substrate, the resultantwafer is heavily bowed as described above. A backside tensile stresslayer balances the stress on the front side of the wafer, yielding aflat wafer. However, such a process requires more expensive, double-sidepolished substrates, more expensive, double-side deposition, morecareful handling, and wafer preparation that must be done afterepitaxial growth but before device fabrication can begin.

BRIEF SUMMARY OF THE INVENTION

In one aspect, a method for controlling bow in wafers which utilizedoped layers is provided. The method comprises depositing asilicon-germanium layer onto a substrate, depositing an undoped bufferlayer onto the silicon-germanium layer, and depositing a boron dopedlayer onto the undoped layer.

In another aspect, a wafer is provided which comprises a substratelayer, a silicon-germanium layer deposited onto the substrate layer, anundoped buffer layer deposited onto the silicon-germanium layer, and aboron doped silicon layer deposited onto the undoped layer.

In still another aspect, a micro-electromechanical system is providedwhich comprises a housing and a micro-machine coupled to the housing. Atleast a portion of the micro-machine comprises boron-doped silicon thathas been etched from a wafer which comprises a substrate layer, asilicon-germanium layer deposited onto the substrate layer, an undopedbuffer layer deposited onto the silicon-germanium layer, and asilicon-boron layer deposited onto the undoped buffer layer.

In yet another aspect, a gyroscope is provided which comprises at leastone proof mass, at least one motor drive comb, and at least one motorpick-off comb. The proof masses, motor drive combs, and motor pick-offcombs comprise boron-doped silicon that has been etched from a waferwhich comprises a substrate layer, a silicon-germanium layer depositedonto the substrate layer, an undoped buffer layer deposited onto thesilicon-germanium layer, and a silicon-boron layer deposited onto theundoped buffer layer.

In another aspect, a method for reducing and controlling bow in waferswhich are formed from stacked and doped silicon layers is provided. Themethod comprises creating stress-relieving dislocations within thestacked silicon layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the problem of bowing when a boron doped layer isdeposited on a substrate for construction of a wafer.

FIG. 2 illustrates one solution for the bowing problem of FIG. 1.

FIG. 3 illustrates another solution for the bowing problem of FIG. 1.

FIG. 4 illustrates construction of a wafer which controls an amount ofbowing.

FIG. 5 is a flowchart illustrating a method for producing the waferillustrated in FIG. 4.

FIG. 6 is a side view of an example MEMS device utilizing the waferdescribed in FIG. 4.

FIG. 7 is a schematic view of a MEMS gyroscope which can be producedutilizing the wafer described in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the problem of bowing in construction of a wafer 10when a heavily boron doped silicon layer 12 is deposited on a siliconsubstrate 14. In the embodiment shown, a heavily boron doped layer 12 isdeposited on silicon substrate 14. In one known scenario, boron dopantconcentration in doped layer 12 is >1×10²⁰ cm⁻³. At such aconcentration, boron atoms, which are smaller than silicon atoms, causea shrinkage of the silicon lattice within boron doped silicon layer 12.Thus doped layer 12 has a high tensile strain compared to substrate 14,causing wafer 10 to bow. The bow is severe enough that many pieces offabrication equipment cannot handle wafers 10 for further fabricationprocesses, for example, fabrication of a MEMS gyroscope.

FIG. 2 illustrates a boron-germanium co-doping solution for the bowingproblem of FIG. 1. Boron-germanium co-doping is one of the known methodsfor controlling wafer bow. Referring to the Figure, a boron-germaniumco-doped layer 20 is deposited on substrate 14, resulting in arelatively flat wafer 22. Germanium is an atom that is larger thansilicon, but is in the same column of the periodic table as silicon.Therefore, if the silicon is doped with germanium and boron at the sametime, the larger germanium atoms compensate for the smaller boron atoms,but do not create any electronic changes. As stated above, negativeconsequences for reducing wafer bow through boron-germanium co-dopinginclude degradation of the mechanical properties of silicon andsegregation at an interface 24 between substrate 14 and co-doped layer20. This segregation creates undesirable stress gradients in co-dopedlayer 20.

FIG. 3 illustrates another known solution for the bowing wafer problemdescribed with respect to FIG. 1. Referring to FIG. 3, heavily borondoped silicon layer 12 is deposited on a first side 30 of siliconsubstrate 14. A backside tensile stress layer 32, which applies a stresssimilar to that of doped layer 12, is deposited on a second side 34 ofsilicon substrate 14 to control bowing of wafer 36. Applying layer 32 tosecond side 34 of substrate 14, with a stress similar to that on firstside 34 of substrate 14, will cause wafer 36 to flatten, providing ahigher quality doped material for use in manufacturing operation, forexample, the MEMS gyroscopes mentioned above. Backside layer 32, in oneembodiment, is heavily boron doped silicon, like layer 12. Inalternative embodiments, backside layer 32 is a different material.While utilization of backside tensile stress layer 32 is a workablesolution for the problem of wafer bowing, control over the process isnot as good as control over the germanium co-doping process. Further,deposition of backside layer 32 involves significantly more processingand/or deposition steps than with germanium co-doping. Also, double-sidepolished substrates, which are more expensive than ordinary single-sidepolished substrates, are required for deposition of backside layer 32.

FIG. 4 illustrates an improvement in construction of a wafer 50 whichalso controls an amount of bowing. In construction of wafer 50, agermanium-silicon layer is used to compensate for stress created by aheavily boron-doped layer, but the germanium doping and the boron dopingare done in spatially separated layers. Referring specifically to FIG.4, a silicon-germanium (Si—Ge) epitaxial layer 52 is deposited onsilicon substrate 14. Deposition of Si—Ge layer 52 onto siliconsubstrate 14 causes the resulting wafer to bow in a direction concave toa back surface 54 of silicon substrate 14 (opposite to the bowingillustrated in FIG. 1). However, the bowing is limited. As stressbetween silicon substrate 14 and Si—Ge layer 52 builds, it causesdislocations to form in Si—Ge layer 52. The dislocations result inrelaxation of the stress. The relaxation results in reduced bowing. Athin, undoped buffer layer 56 of silicon is deposited on top of Si—Gelayer 52 to prevent mixing of Si—Ge layer 52 and a silicon-boron (Si—B)layer 58 which is deposited on buffer layer 56. In one embodiment, layer58 is a very heavily doped silicon-boron etch stop layer. In a specificembodiment, concentration of boron in Si—B layer 58 is between about 0.1percent and about 1.0 percent. Silicon-boron layer 58 is grown attemperatures between about 1000° C. and about 1200° C., with boronconcentration between about 5×10¹⁹ and about 5×10²⁰ cm⁻³, and athickness between 5 and 50 μm.

When Si—B layer 58 is deposited above buffer layer 56 and Si—Ge layer52, a stress buildup is created in an opposite direction from the stresscreated by deposition of Si—Ge layer 52. The stress attempts to makewafer 50 bow concave toward a front surface 60. Again, dislocations aregenerated in SiGe layer 52 that relax the built up stress and reducesbowing of wafer 50. To restate, bow is reduced by relaxing the stressesinstead of by balancing the stress, as is done in known methods.

Generation of dislocations can lead to surface roughness of wafer 50. Anappropriate range of deposition parameters, for example, temperature,germanium concentration, and thickness for Si—Ge layer 52 and Si—B layer58, have been determined that simultaneously yield low bowing and smoothsurfaces 54, 60 for wafer 50. Silicon-germanium layer 52 is grown attemperatures between about 600° C. and about 1000° C. with germaniumconcentration between about 2 and about 20 atomic percent.Silicon-germanium layer 52 is grown with a thickness between about 0.1and about 5.0 μm.

Germanium-doped layer 52 is utilized to compensate the stress created byboron-doped layer 58, but the germanium doping and the boron doping aredone in spatially separated layers. The thickness and concentration ofsilicon-germanium layer 52 can be adjusted to achieve compensation forboron-doped layer 58. Because silicon-germanium layer 52 is nowessentially part of the substrate that will eventually be etched,germanium concentration must be low enough that it does not interferewith the etching.

The layer in which the device (e.g. a MEMS gyroscope) is formed, is nowonly doped with boron, eliminating the material degradation problemsinherent in the above described boron-germanium co-doping. Non-dopedbuffer layer 56 separates the germanium from the boron, so none of theeffects of segregation are present. Further, all of the deposition isdone on a front side of the silicon substrate, eliminating the need toturn the wafer over for backside processing. Such wafer constructionalso avoids potential front side damage from the backside processingsteps, and allows the use of lower-cost, single-side polishedsubstrates.

FIG. 5 is a flowchart 70 illustrating a method for producing waferssimilar to wafer 50 (shown in FIG. 4). A silicon-germanium epitaxiallayer 52 is deposited 72 onto a silicon substrate wafer 14. An undopedsilicon buffer layer 56 is deposited 74 onto silicon-germanium epitaxiallayer 52. A very heavily boron-doped silicon layer 58 is deposited 76onto undoped silicon buffer layer. As stresses build in boron-dopedsilicon layer 58, dislocations are generated in silicon-germanium layer52, causing it to relax with respect to silicon substrate 14 orboron-doped layer 58. The relaxation reduces the wafer bow to lowlevels. The method illustrated by flowchart 70 contrasts earlier methodsof bow control, for example, a backside tensile stress layer (shown inFIG. 3), where additional layers balance the stress on the two sides ofthe wafer. Whereas, in the wafer and method illustrated in FIGS. 4 and 5respectively, stresses are relaxed (i.e., reduced) on one side of thewafer, and therefore, a balancing layer (backside layer 32) on the otherside of the substrate is not needed.

FIG. 6 is a diagram of one embodiment of a Micro-ElectromechanicalSystem (MEMS) 100 constructed utilizing wafer 50 (shown in FIG. 4).While FIG. 6 illustrates a MEMS gyroscope (as described further withrespect to FIG. 7), other sensors may also be constructed utilizingwafer 50 as well. For example, an accelerometer, a resonator, a pressuresensor, a temperature sensor, an air flow sensor, and any other deviceusing a heavily boron doped layer which is not necessarily bonded toglass are examples of devices which can be constructed utilizing wafer50. Therefore, it should be understood that MEMS 100 illustrated in FIG.6 and described herein are set forth for purposes of example only, andother arrangements and elements can be used instead and some elementsmay be omitted altogether, depending on manufacturing and/or consumerpreferences.

MEMS 100 includes a housing 102 to which a cover (not shown) iseventually attached in order to form a sealed cavity. Electrical leads106 provide electrical connections to a micro-machine 108 chip which iscoupled to housing 102. Micro-machine chip 108 includes a micro-machine110. At least a portion of micro-machine 110 includes boron-dopedsilicon 112 that has been etched from boron doped layer 58 (shown inFIG. 4). For example, in the case of a MEMS tuning fork gyroscope,silicon 112 includes, proof masses 114, motor drive combs 116, and motorpick-off combs 118. Micro-machine 110 further includes sense plates 120which form parallel plate capacitors with proof masses 114. In oneembodiment, sense plates 120 are metal films that have been depositedand patterned. A machine cover 122 is coupled to micro-machine chip 108using multiple bonds, for example, bonds to motor drive combs 116 andmotor pick-off combs 118. Such a bonding configuration for machine cover122, coupled with cavities machined into machine cover 122 provide anopen space 124 between machine cover 122 and micro-machine chip 108.Open space 124 allows components of micro-machine 110 an ability to movefreely. For example, proof masses 114 may be movably coupled tomicro-machine chip 108 and therefore may oscillate within open space124.

MEMS 100 may comprise more or fewer components than described. Forinstance, while two electrical contacts 106 are illustrated, thoseskilled in the art will recognize that a MEMS may comprise more than twocontacts and/or extruding pins as well. Additionally, more or fewermembers may be present in MEMS 100 other than those components abovedescribed. Further, components of MEMS 100 may comprise multiplefunctions. Machine cover 122 of MEMS 100 may be comprised of a materialsuch as silicon, glass or a ceramic material. Micro-machine 110 may beany such electromechanical machine used in accordance with MEMS and MEMSbased devices. In addition, alternate packages may be used as well toprovide a housing for MEMS 100.

FIG. 7 is a schematic illustration of a MEMS gyroscope 140 whichillustrates components of such a gyroscope in accordance with thecomponents described in FIG. 6. Gyroscope 140 may utilize a wafer, forexample, wafer 50 (shown in FIG. 4) in construction of certaincomponents, as described above. Referring specifically to the Figure,MEMS gyroscope 140 may include a housing 142 that includes therein atuning fork gyroscope (TFG) 144. Housing 142 may be a plastic package, asmall outline integrated circuit (SOIC) package, a plastic leaded chipcarrier (PLCC) package, a quad flat package (QFP), or other housings asknown in the art. Housing 142 may provide a structure to co-locateelements of TFG 144 and/or locate other elements within a closeproximity of one another within the housing 142. TFG 144, in oneembodiment, is located within a substantially sealed cavity 105 which isformed by bonding cover 104 (shown in FIG. 6) to housing 102 (shown inFIG. 6).

In one embodiment, TFG 144 may include proof masses 114, motor drivecombs 116, motor pick-off combs 118, and sense plates 120 constructedfrom a wafer, for example, wafer 50 (shown in FIG. 4). A pre-amplifier146 may be included within housing 142 and may be electrically connectedor coupled to each proof mass 114 and sense plate 120 combination.Pre-amplifier 146 and TFG 144 may both be formed on a common substrateand, in one embodiment, may be electrically connected. In otherembodiments, pre-amplifier 146 may be electrically connected to proofmasses 114. An output of pre-amplifier 146 may be sent to senseelectronics 148, or alternatively, pre-amplifier 146 may be incorporatedwithin sense electronics 148.

In addition, an output 150 of motor pick-off combs 118 is transferred tofeedback monitors 152. Feedback monitors 152 provide output signals 154to drive electronics 156, which power motor drive combs 116.Alternatively, feedback monitors 152 may be incorporated within driveelectronics 156. MEMS gyroscope 140 may also include a system powersource and other operational electronics, which are not shown in FIG. 7for ease of illustration.

Motor drive combs 116 excite the proof masses 114 using electrostaticforces by applying a voltage to electrodes of proof masses 114. Motorpick-off combs 118 monitor the excitation or oscillation of proof masses114 by monitoring voltage signals on electrodes on proof masses 114.Motor pick-off combs 118 output a feedback signal to feedback monitors152. Feedback monitor 152 provides an output 154 which is input to driveelectronics 156. If proof masses 114 begin to oscillate too fast or tooslow, drive electronics 156 may adjust an oscillation frequency suchthat proof masses 114 vibrate at a resonant frequency. Excitation atsuch a frequency may enable a higher amplitude output signal to begenerated.

While operation of gyroscope 140 is described in entirety, suchoperation is not likely if boron-doped wafers, for example, wafer 10 asshown in FIG. 1, bow during a fabrication stage. As described, suchbowing may be severe enough to require extra machining steps, whichincrease production time and add costs. Utilization of a waferfabrication technique such as embodied in wafer 50 (shown in FIG. 4)provides sensors, that have little or no bowing, and therefore allowsfabrication of gyroscope 140. Such wafers are further usable in othersensor based-devices which are mentioned above.

As stated above with respect to FIG. 2, negative consequences of using agermanium-boron co-doped layer to reduce bow include degradation of themechanical properties of silicon and segregation at the interfacebetween the substrate and the co-doped layer. Therefore, current bowreduction approaches utilize epitaxial deposition on both sides of asilicon substrate, as described above with respect to FIG. 3. The methodfor wafer bow reduction described with respect to FIGS. 4 and 5 requiredeposition of layers on only one side of a silicon substrate, whichreduces costs associated with of deposition by approximately half overthe costs of a backside tensile layer. The illustrated method thereforeallows utilization of less expensive, silicon substrates, as only asingle side of the substrate has to be polished.

In addition, in the backside tensile layer approach, since epitaxiallayers are grown (deposited) on both sides of the silicon substrate, aprotective layer has to be deposited over the silicon-boron layer. Thisprotective layer must be stripped off the silicon-boron layer prior toproduct fabrication (i.e. MEMS gyroscopes), costing more time and moneyin wafer preparation. Finally, thickness of the epitaxial layers aremore easily controlled utilizing the wafer and methods illustrated inFIGS. 4 and 5 respectively, as there is less total time spent at hightemperature, thus sharpening the interface between the epitaxial layerand the undoped buffer layer by reducing diffusion. Therefore theadvantages described herein include, lower wafer cost due to singlesided epitaxial layer deposition, elimination of post-growth waferpreparation, and an improved ability to measure and control thickness.It is contemplated that the wafer construction method and resultantwafers described herein will find utilization in a variety of MEMSproducts, including, but not limited to, MEMS inertial guidanceproducts, gyroscopes, and accelerometers.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1.-7. (canceled)
 8. A wafer comprising: a substrate layer; asilicon-germanium layer deposited onto said substrate layer; an undopedbuffer layer deposited onto said silicon-germanium layer; and a borondoped silicon layer deposited onto said undoped buffer layer. 9.(canceled)
 10. A wafer according to claim 8 wherein said undoped bufferlayer has a thickness between about 0.1 and about 5.0 micrometers.
 11. Awafer according to claim 8 wherein said silicon-boron layer has athickness between about 5.0 and about 50.0 micrometers.
 12. (canceled)13. A wafer according to claim 8 wherein a concentration of boron insaid silicon-boron layer is between about 5×10¹⁹ and about 5×10²⁰ cm⁻³.14. A micro-electromechanical system (MEMS) comprising: a housing; amicro-machine coupled to said housing, at least a portion of saidmicro-machine comprising boron-doped silicon that has been etched from awafer which comprises a substrate layer, a silicon-germanium layerdeposited onto said substrate layer, an undoped buffer layer depositedonto said silicon-germanium layer, and a silicon-boron layer depositedonto said undoped buffer layer.
 15. A MEMS according to claim 14 whereinboron-doped silicon comprises proof masses, motor drive combs, and motorpick-off combs for a tuning fork gyroscope.
 16. A MEMS according toclaim 14 wherein a concentration of boron in said silicon-boron layer isbetween about 0.1 percent and about 1.0 percent.
 17. A MEMS according toclaim 14 wherein said silicon-boron layer has a thickness between about5.0 and about 50.0 micrometers.
 18. A MEMS according to claim 14 whereinsaid micro-machine comprises one or more of an accelerometer, aresonator, a pressure sensor, a temperature sensor and an air flowsensor.
 19. A gyroscope comprising: at least one proof mass; at leastone motor drive comb; and at least one motor pick-off comb, said proofmasses, said motor drive combs, and said motor pick-off combs comprisingboron-doped silicon that has been etched from a wafer which comprises asubstrate layer, a silicon-germanium layer deposited onto said substratelayer, an undoped buffer layer deposited onto said silicon-germaniumlayer, and a silicon-boron layer deposited onto said undoped bufferlayer. 20.-22. (canceled)